Conventionally, there is proposed a resin film having a copper foil on one face thereof (one-face-copper-foil-attached resin film) configured by providing a copper foil and a prepreg or a copper foil and a resin film as a material used for manufacturing a multilayer wiring substrate by integrally laminating the film on an inner material layer having a wiring formed thereon. Since the copper foil of the one-face-copper-foil-attached resin film is thin, fine wirings can be formed, and hence a high density, thin and miniaturized wirings can be formed. On the other hand, as a via hole connecting wirings of upper and lower layers, a via hole having a minute diameter can be easily formed with increased introduction of a excimer laser and a laser processing machine using third and fourth harmonics of YAG, and with development of a direct laser method using a CO2 laser.
Also, by applying these, a method for manufacturing a multilayer wiring substrate has been actively used, the multilayer wiring substrate being manufactured in such a manner that a prepreg and a metal foil provided on the prepreg are laminated integrally on an inner layer material on which a wiring is formed, and that a hole for via hole is provided by a laser and is filled with an electrolytic plating layer formed by using an electrolytic filled plating solution (hereinafter simply referred to as “electrolytic filled plating layer”) after formation of a base electroless plating layer. In this case, in order to suppress plating voids (hereinafter simply referred to as “voids”) formed in the via and each having a via diameter which is almost equal to the thickness of the insulating layer, that is, has an aspect ratio of about 1, an electrolytic plating method in which electrolytic plating is performed at a low current density for a long time, or an electrolytic plating method in which current density is controlled stepwise are proposed (Patent Literature 1). Further, as a method for filling a via hole, there is proposed a method in which an electrolytic plating layer is separately formed twice from a viewpoint of surface smoothness (Patent Literature 2).